# 6.9* Einstein’s Special Theory of Relativity

As a consequence of physical experiments performed in the latter half of the nineteenth century (most notably the Michelson-Morley experiment of 1887), physicists concluded that the results obtained in measuring the speed of light c are independent of the velocity of the instrument used to measure it. For example, suppose that while on Earth, an experimenter measures the speed of light emitted from the sun and finds it to be 186,000 miles per second. Now suppose that the experimenter places the measuring equipment in a spaceship that leaves Earth traveling at 100,000 miles per second in a direction away from the sun. A repetition of the same experiment from the spaceship yields the same result: Light is traveling at 186,000 miles per second relative to the spaceship, rather than 86,000 miles per second as one might expect!

This revelation led to a new way of relating coordinate systems used to locate events in space-time. The result was Albert Einstein’s special theory of relativity. In this section, we develop via a linear algebra viewpoint the essence of Einstein’s theory.

The basic problem is to compare two different inertial (nonaccelerating) coordinate systems S and  that are in motion relative to each other under the assumption that the speed of light is the same when measured in either system. We assume that  moves at a constant velocity in relation to S as measured from S.

To simplify matters, we assume that the two coordinate systems have parallel axes and share the same x-axis, and that the motion of  relative to S is along this common axis. (See Figure 6.8.)

We also suppose that there are two clocks, C and , placed in space so that C is stationary relative to S and  is stationary relative to . These clocks give readings that are real numbers in units of seconds. They are calibrated so that at the instant the origins of S and  coincide, both give the reading of 0.

Given any event p (something whose position and time of occurrence can be described), we may assign a set of space-time coordinates to it. For example, if p is an event that occurs at position (x, y, z) relative to S and at time t as read on clock C, we can assign to p the set of coordinates



This ordered 4-tuple is called the space—time coordinates of p relative to S and C. Likewise, p has a set of space-time coordinates



relative to  and .

Because motion is along a common x-axis, which lies in a common xy-plane, the third component of the space-time coordinates of p is always zero. Thus we consider only the first, second, and fourth coordinates of p, and write



to denote the space-time coordinates of an event p relative to S and , respectively.

As we have mentioned, our unit of time is the second. Our measure of an object’s velocity v is the ratio of its velocity (expressed in miles per second) to the speed of light expressed in the same units (which is approximately 186,000 miles per second). For example, if  is moving at 18,600 miles per second relative to S and the speed of light, c, is 186,000 miles per second, the velocity of  relative to S, v, would have a value of . For this reason, the speed of light c has the value 1.

For a fixed velocity v, let  be the mapping defined by



where



are the space-time coordinates of the same event with respect to S and C and with respect to  and , respectively.

In what follows, we make four assumptions:

1. The origin of  moves in the positive direction of the common x-axis relative to S at a constant velocity of .

2. The origin of S moves in the negative direction of the common x-axis relative to  at the constant velocity of .

3.  is a linear isomorphism.

4. The speed of any light beam, when measured in either S or , using the clocks C in S and  in , is always .

Since motion is strictly along the x-axis and we assume that the y-axis is unaffected, we have that for any x, y, and t, there exist  and  such that



Our goal in this section is to calculate the matrix representation of  with respect to the standard basis for .

# Theorem 6.39.

Consider , the standard ordered basis for . Then

1. (a) 

2. (b)  maps  into itself.

3. (c)  maps  into itself.

# Proof.

Parts (a) and (b) follow immediately from the equations above. For  and ,



and hence (c) follows.

Suppose that, at the instant the origins of S and  coincide, a light flash is emitted from their common origin. The event of the light flash when measured either relative to S and C or relative to  and  has space-time coordinates



Let P be the set of all events in the xy-plane whose space-time coordinates



relative to S and C are such that the flash is observable in the common xy-plane at the point (x, y) (as measured relative to S) at the time t (as measured on C). Let us characterize P in terms of x, y, and t. Since the speed of light is 1, at any time  the light flash is observable from any point in the plane whose distance to the origin of S (as measured on S) is . These are precisely the points in the xy-plane with , or  Hence an event lies in P if and only if its space-time coordinates



relative to S and C satisfy the equation . Since the speed of light when measured in either coordinate system is the same, we can characterize P in terms of the space-time coordinates relative to  and  similarly: An event lies in P if and only if, relative to  and , its space-time coordinates



satisfy the equation .

Let



# Theorem 6.40.

If  for some , then



# Proof.

Let



and suppose that .

CASE 1. . Since , the vector w gives the coordinates of an event in P relative to S and C. Because



are the space-time coordinates of the same event relative to  and , the discussion preceding Theorem 6.40 yields



Thus , and the conclusion follows.

CASE 2. . The proof follows by applying case 1 to .

We now proceed to deduce information about . Let



Clearly  is an orthogonal basis for the span of . The next result tells us even more.

# Theorem 6.41.

There exists a nonzero scalar a such that  and .

# Proof.

Because  by Theorem 6.40. Thus  is orthogonal to . Since  is an orthogonal basis for span  and each of , and  maps this span into itself, it follows that  must be a multiple of , that is,  for some scalar a. Since  and A are invertible, so is . Thus .

Similarly, there exists a nonzero scalar b such that .

Finally, we show that . Since , we have



So .

Actually, , as we see in the following result.

For the rest of this section let , where  is the standard ordered basis for .

# Theorem 6.42.

Given , as defined above,

1. (a) 

2. (b) 

# Proof.

Since



it follows from Theorem 6.41 that



Furthermore, , and hence



Let



Then , and hence by Theorem 6.40



Thus . As a consequence, . This proves (a). Part (b) now follows.

Now consider the situation 1 second after the origins of S and  have coincided as measured by the clock C. Since the origin of  is moving along the x-axis at a velocity v as measured in S, its space-time coordinates relative to S and C are



Similarly, the space-time coordinates for the origin of  relative to  and  must be



for some . Thus we have

 (18)
 (19)

But also

 (20)

Combining (19) and (20), we conclude that , or

 (21)

Thus, from (18) and (21), we obtain

 (22)

Next, recall that the origin of S moves in the negative direction of the -axis of  at the constant velocity  as measured from . Consequently, 1 second after the origins of S and  have coincided as measured on clock C, there exists a time  as measured on clock  such that

 (23)

From (23), it follows in a manner similar to the derivation of (22) that

 (24)

Hence, from (23) and (24),

 (25)

The following result is now easily proved using (22), (25), and Theorem 6.39.

# Theorem 6.43.

Let  be the standard ordered basis for  . Then



# Time Contraction

A most curious and paradoxical conclusion follows if we accept Einstein’s theory. Suppose that an astronaut leaves our solar system in a space vehicle traveling at a fixed velocity v as measured relative to our solar system. It follows from Einstein’s theory that, at the end of time t as measured on Earth, the time that passes on the space vehicle is only . To establish this result, consider the coordinate systems S and  and clocks C and  that we have been studying. Suppose that the origin of  coincides with the space vehicle and the origin of S coincides with a point in the solar system (stationary relative to the sun) so that the origins of S and  coincide and clocks C and  read zero at the moment the astronaut embarks on the trip.

As viewed from S, the space-time coordinates of the vehicle at any time  as measured by C are



whereas, as viewed from , the space-time coordinates of the vehicle at any time  as measured by  are



But if two sets of space-time coordinates



are to describe the same event, it must follow that



Thus



From the preceding equation, we obtain , or

 (26)

This is the desired result.

A dramatic consequence of time contraction is that distances are contracted along the line of motion (see Exercise 7).

Let us make one additional point. Suppose that we choose units of distance and time commonly used in the study of motion, such as the mile, the kilometer, and the second. Recall that the velocity v we have been using is actually the ratio of the velocity using these units with the speed of light c, using the same units. For this reason, we can replace v in any of the equations given in this section with the ratio v/c ,where v and c are given using the same units of measurement. Thus, for example, given a set of units of distance and time, (26) becomes



# Exercises

1. Complete the proof of Theorem 6.40 for the case .

2. For



show that

1. (a)  is an orthogonal basis for 

2. (b)  is -invariant.

3. Derive (24), and prove that

 (25)

Hint: Use a technique similar to the derivation of (22).

4. Consider three coordinate systems  and  with the corresponding axes ( and ) parallel and such that the  and -axes coincide. Suppose that  is moving past S at a velocity  (as measured on S),  is moving past  at a velocity  (as measured on ), and  is moving past S at a velocity  (as measured on S), and that there are three clocks  and  such that C is stationary relative to  is stationary relative to , and  is stationary relative to . Suppose that when measured on any of the three clocks, all the origins of , and  coincide at time 0. Assuming that  (i.e., ), prove that



Note that substituting  in this equation yields . This tells us that the speed of light as measured in S or  is the same. Why would we be surprised if this were not the case?

5. Compute . Show . Conclude that if  moves at a negative velocity v relative to S, then , where  is of the form given in Theorem 6.43. Visit goo.gl/9gWNYu for a solution.

6. Suppose that an astronaut left Earth in the year 2000 and traveled to a star 99 light years away from Earth at 99% of the speed of light and that upon reaching the star immediately turned around and returned to Earth at the same speed. Assuming Einstein’s special theory of relativity, show that if the astronaut was 20 years old at the time of departure, then he or she would return to Earth at age 48.2 in the year 2200. Explain the use of Exercise 4 in solving this problem.

7. Recall the moving space vehicle considered in the study of time contraction. Suppose that the vehicle is moving toward a fixed star located on the x-axis of S at a distance b units from the origin of S. If the space vehicle moves toward the star at velocity v, Earthlings (who remain “almost” stationary relative to S) compute the time it takes for the vehicle to reach the star as . Due to the phenomenon of time contraction, the astronaut perceives a time span of . A paradox appears in that the astronaut perceives a time span inconsistent with a distance of b and a velocity of v. The paradox is resolved by observing that the distance from the solar system to the star as measured by the astronaut is less than b.

Assuming that the coordinate systems S and  and clocks C and  are as in the discussion of time contraction, prove the following results.

1. (a) At time t (as measured on C), the space-time coordinates of the star relative to S and C are


2. (b) At time t (as measured on C), the space-time coordinates of the star relative to  and  are


3. (c) For



we have .

This result may be interpreted to mean that at time  as measured by the astronaut, the distance from the astronaut to the star, as measured by the astronaut, (see Figure 6.9) is


4. (d) Conclude from the preceding equation that

(1) the speed of the space vehicle relative to the star, as measured by the astronaut, is ;

(2) the distance from Earth to the star, as measured by the astronaut, is .

Thus distances along the line of motion of the space vehicle appear to be contracted by a factor of .